diff --git a/packages/core/src/page.js b/packages/core/src/page.js
index 8920b3a4a..1a5db7aac 100644
--- a/packages/core/src/page.js
+++ b/packages/core/src/page.js
@@ -25,22 +25,40 @@ export const DEFAULT_WAIT_FOR_CUSTOM_ELEMENTS_TIMEOUT = 500;
 // IMPORTANT: this body is intentionally ES5 — it is evaluated in the
 // page's realm and must work in any browser the page targets. Don't
 // "modernize" with arrow functions, let/const, or optional chaining.
+//
+// serializeFunction() injects the Percy helpers object as `arguments[0]`, so
+// the caller's first argument is `arguments[1]`.
 export const WAIT_FOR_CUSTOM_ELEMENTS_BODY = `
-  var deadline = Date.now() + (arguments[0] || 500);
+  var timeoutMs = typeof arguments[1] === 'number' && isFinite(arguments[1]) && arguments[1] > 0
+    ? arguments[1]
+    : 500;
+  var deadline = Date.now() + timeoutMs;
   return new Promise(function(resolve) {
+    var settled = false;
+    function finish() {
+      if (settled) return;
+      settled = true;
+      resolve();
+    }
+    setTimeout(finish, timeoutMs);
     function tick() {
+      if (settled) return;
       var undef = document.querySelectorAll(":not(:defined)");
-      if (!undef.length) return resolve();
-      if (Date.now() >= deadline) return resolve();
+      if (!undef.length) return finish();
+      if (Date.now() >= deadline) return finish();
       var names = {};
       for (var i = 0; i < undef.length; i++) names[undef[i].localName] = true;
       var promises = Object.keys(names).map(function(n) {
-        return window.customElements.whenDefined(n).catch(function(){});
+        try {
+          return window.customElements.whenDefined(n).catch(function(){});
+        } catch (e) {
+          return Promise.resolve();
+        }
       });
       Promise.race([
         Promise.all(promises),
         new Promise(function(r) { setTimeout(r, 100); })
-      ]).then(tick);
+      ]).then(tick, finish);
     }
     tick();
   });
diff --git a/packages/core/test/percy.test.js b/packages/core/test/percy.test.js
index 0b93c1be3..992d1b0ab 100644
--- a/packages/core/test/percy.test.js
+++ b/packages/core/test/percy.test.js
@@ -4,7 +4,11 @@ import Percy from '@percy/core';
 import Pako from 'pako';
 import DetectProxy from '@percy/client/detect-proxy';
 import { validateSnapshotOptions } from '../src/snapshot.js';
-import { Page, WAIT_FOR_CUSTOM_ELEMENTS_BODY } from '../src/page.js';
+import {
+  Page,
+  WAIT_FOR_CUSTOM_ELEMENTS_BODY,
+  DEFAULT_WAIT_FOR_CUSTOM_ELEMENTS_TIMEOUT
+} from '../src/page.js';
 
 describe('Percy', () => {
   let percy, server;
@@ -216,6 +220,31 @@ describe('Percy', () => {
     ]));
   });
 
+  it('does not hang on a page holding a never-defined custom element', async () => {
+    // PER-10405: an unregistered tag matches `:not(:defined)` forever and its
+    // `customElements.whenDefined()` never resolves, so the wait must fall back
+    // on its own ceiling.
+    server.reply('/', () => [200, 'text/html', (
+      '<p>hi</p><next-route-announcer></next-route-announcer>'
+    )]);
+    await percy.browser.launch();
+    let page = await percy.browser.page();
+    await page.goto('http://localhost:8000');
+
+    let undefinedCount = await page.eval(
+      () => document.querySelectorAll(':not(:defined)').length);
+    expect(undefinedCount).toBe(1);
+
+    let start = Date.now();
+    await expectAsync(page.eval(
+      WAIT_FOR_CUSTOM_ELEMENTS_BODY,
+      DEFAULT_WAIT_FOR_CUSTOM_ELEMENTS_TIMEOUT
+    )).toBeResolved();
+    let elapsed = Date.now() - start;
+
+    expect(elapsed).toBeLessThan(5000);
+  });
+
   it('runs readiness check before serializing when readiness option is set', async () => {
     server.reply('/', () => [200, 'text/html', '<p>Hello Percy!</p>']);